HALIFAX -- Nicholas Butcher says he fatally stabbed Montreal-born yoga instructor Kristin Johnston by accident, and then tried to kill himself by cutting off his hand with a mitre saw.

Butcher, testifying Friday at his second-degree murder trial, said the pair had been sleeping in a bed inside Johnston's Halifax-area home on March 26, 2016, when he awoke to someone stabbing him in the throat with a knife.

He said it was dark and he couldn't see who it was, but he flipped the attacker over, grabbed the knife and stabbed them.

"I reached over and I turned on the lamp, and the person that was beneath me was Kristin and she was dead," a visibly distraught Butcher told the 14-member jury in Nova Scotia Supreme Court.

"I just killed her by accident, just not even knowing what's going on."

Butcher said he then tried to slit his wrist with the knife and razor blades, but the suicide attempts failed.

"I thought to myself, 'I have to die, too,"' said the 36-year-old law school graduate.

That's when he used the mitre saw to cut off his right hand.

Earlier in the trial, the jury heard that Butcher called 911 and told the dispatcher he had killed his girlfriend, tried to kill himself and had cut off his hand.

As well, the court heard Butcher and Johnston, 32, were in a relationship and were living together. However, a friend of Johnston's testified she had broken up with him only hours before her death.

On Friday, Butcher testified that he was turning off music playing on Johnston's laptop when he noticed messages from a man whose name and address he didn't recognize.

He said he assumed Johnston had gone to a party.

"I decided to stop by and see what was going on," said Butcher.

When he arrived at the apartment, Johnston told him she was "too drunk to talk," but he said she also gave him a kiss and told him she loved him.

After he left, Butcher said he decided to wait in his car in case Johnston needed a ride.

Butcher said he tried to call Johnston three times, but she didn't answer, and he became so concerned that he decided to return to the apartment.

"About halfway up the stairs, I heard Kristin moaning," he said.

When he found Johnston in bed with another man, Butcher said he "wasn't leaving without her."

On the drive back to Johnston's house, she was yelling at him and calling him names, he said. But she then started crying, telling him he didn't deserve the way she treated him, Butcher testified.

However, once they arrived at the house, Johnston became angry again.

"I remember thinking, 'This isn't Kristin,"' he said. "I remember almost being afraid."

The two eventually went to bed, he said, recalling that he later awoke to someone stabbing him.

Under cross-examination, Crown lawyer Carla Ball said Butcher told the 911 dispatcher six times that he had killed his girlfriend, but never mentioned anything about it being an accident, or that she had attacked him.

"I wasn't giving a narrative of what happened that night," Butcher replied. "I think I was in shock."

Ball noted that Butcher didn't have a problem clarifying his address for the dispatcher multiple times.

Police found Johnston's body on a blood-soaked bed next to a steak knife.

Medical examiner Dr. Marnie Wood testified that Johnston's death was caused by sharp force injuries. Wood also said Johnston had "defensive injuries" on her hands and fingers.

Dr. Jonathan Trites, a head and neck surgeon, testified that Butcher had 13 sharp wounds on his neck.